Problem: Complete the recursive formula of the arithmetic sequence $-17, -8, 1, 10,...$. $a(1)=$
Explanation: The first term is $-17$ and the common difference is $9$. ${+9\,\curvearrowright}$ ${+9\,\curvearrowright}$ ${+9\,\curvearrowright}$ $-17,$ $-8,$ $1,$ $10,...$ This is the recursive formula of $-17, -8, 1, 10,...$ $\begin{cases} a(1)=-17 \\\\ a(n)=a(n-1)+9 \end{cases}$